Mono-and dimethyl derivatives (II) and (III) of Yang's biradical (I) were prepared, and the methyl-substitution effects on the molecular symmetry and spin-density distribution of Yang's biradical have been studied. Resolved hyperfine structures were observed in the ESR spectra of liquid solutions of the biradicals (II) and (III), giving the hyperfine splitting values theoretically expected for their triplet states. In a fluid solution, the biradical molecules ...